Horses were expensive and could only be used efficiently by men who were expert riders and who knew how use a horse in a combat situation.

Even more, with the arrival the Europeans in the fifteenth century, West Africa was introduced to guns and gunpowder.

These, too, were expensive required trained soldiers to make good use of them.
While the new military technology had increased the ruler's freedom from popular control, it made him increasingly dependent on and subject to European interests.

The African ruler's desire for guns and the European's desire for slaves went hand in hand..